Cell membrane is made up of two sheets of phospolipid. The phospolipid has a hydrophilic head and a hydrophobic tail. This allows smaller molecules and water molecules to go freely in and out of the cell through a process called passive transport. Presence of embedded globular proteins helps regulate larger molecules which are needed by the cell to pass through the semi-permeable membrane through a process called active transport. Active transport uses energy to transfer molecules in and out of the cell while passive transfer doesn't.
The gradient of concentration or the value of concentration itself inside the cell is called tonicity.! For more info:. It's a fluid mosiac with porins that allow some things in, but does not allow larger things to go through. The fluid mosaic model says the membrane is movable. How do lipid bilayers form? How do ions cross the lipid bilayer? How can a molecule be hydrophobic?
How do cells regulate ion movement? See all questions in Cell Membrane. Impact of this question views around the world. Proteins: The proteins form gates, channels, or pores which provide selective permeability to the membrane. It gives signals to the molecules according to the requirement of the cell.
The membrane proteins are of two types. The proportion of proteins, carbohydrates, and lipids in the selectively permeable membrane varies according to the type of cell.
However, the proportion of lipids and protein varies in different membranes. A phospholipid mainly consists of three components:. Each phospholipid is amphipathic which means it has a hydrophilic head and two hydrophobic tails. The hydrophilic head faces outside and the hydrophobic tail faces inside. Another lipid component is cholesterol. It consists of four fused carbon rings. It is situated alongside phospholipid. Carbohydrates : On the exterior surface of a selectively permeable membrane, carbohydrates are found.
They are either attached to the proteins or lipids and form glycoproteins and glycolipids respectively. The selectively permeable membrane also refers to the cell membrane, providing anchoring to the cytoskeleton. The cytoskeleton provides shape to the cell and in attaching the extracellular matrix and other cells to hold them together and from tissues. The selectively-permeable cell membrane protects the cell.
It maintains the balance in the internal environment of the cell by transporting the toxic substances outside the cell and bringing the nutrients inside the cell. Selectively-permeable membranes help in maintaining the integrity of the intracellular environment as compared to the extracellular environment. It forms a barrier between the cellular organelles and the outside environment. In eukaryotic cells , a selectively-permeable membrane is essential for the transport of molecules across the nuclear membrane.
In transcription, not only the specific nucleotides , proteins, and nucleic acids must be transported into the nucleus efficiently and specifically but also the products must be exported on time. The environment of the nucleus is distinguished from the cytoplasm. Selectively-permeable membrane functions effectively in maintaining equilibrium among these so that the overall cellular function is achieved more efficiently.
Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so.
Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes. Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX.
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